Polymeric aromatic carbamic acid esters (polyurethanes) such as diphenylmethane dicarbamates particularly the diethyl ester thereof, and the related higher homologs, polymethylene polyphenyl carbamates, ethyl esters thereof have become increasingly important products particularly, for use in the preparation of the commercially valuable diphenylmethane diisocyanates and mixtures of diisocyanates and the polymethylene polyphenyl isocyanates by the decomposition of such polymeric aromatic carbamic acid esters in a suitable solvent as shown in Rosenthal et al., U.S. Pat. Nos. 3,962,302, June 8, 1976 and 3,919,279, Nov. 11, 1975 incorporated herein by reference.
A proposed prior art process for the preparation of polymeric aromatic carbamic acid esters (polyurethanes) is disclosed in Klauke et al., U.S. Pat. No. 2,946,768 and involves the condensation of aryl carbamic acid esters with carbonyl compounds in a dilute aqueous acid condensation medium. However, in such process the carbonyl compound such as formaldehyde tends to react at the nitrogen of the carbamate to produce along with desired polyurethanes, varying amounts, i.e., generally between 15 percent and 50 percent by weight, the undesirable N-(alkoxycarbonyl)-phenylaminomethylphenyl compounds which includes the various dimers, trimers, tetramers, etc. of such compounds (also referred to herein as "N-benzyl" compounds). Attempts to prepare mono or diisocyanates and polyisocyanates or to otherwise use the mixture containing the undesired "N-benzyl" compounds, which cannot be converted to an isocyanate by pyrolysis, and polyurethanes presents many problems. However, the undesired N-benzyl compounds may be catalytically rearranged to a desired polyurethane in accordance with the teachings of Shawl et al., U.S. Pat. No. 4,146,727, Mar. 27, 1979 and incorporated herein by reference. Accordingly, a product mixture from a condensation as disclosed in aforementioned U.S. Pat. No. 2,946,768 containing diurethanes and polyurethanes, N-benzyl compounds, unreacted alkylphenylcarbamates and other by-products such as amines may be contacted at temperatures of from about 50.degree. C. to 170.degree. C. with a protonic acid medium having a strength at least equal to a 75 percent sulfuric acid such as concentrated sulfuric acid or an acid medium comprising a Lewis acid having a concentration of at least 0.5 percent by weight based on the total reaction mixture, while maintaining a minimum amount of water in the system, to catalytically convert or rearrange said N-benzyl compounds to mono and dicarbamates and polymethylene polyphenyl carbamates.
Shawl, U.S. Pat. No. 4,172,948, Oct. 30, 1979, incorporated herein by reference, discloses a similar rearrangement of N-benzyl compounds which may be achieved by use of anhydrous hydrogen chloride under super atmospheric pressure.
Condensation of aryl carbamic acid esters especially ethylphenylcarbamate using formaldehyde, paraformaldehyde or trioxane may also be conducted with organic sulfonic acids. Shawl, U.S. Pat. No. 4,162,362, July 24, 1979, incorporated herein by reference, teaches that condensation in the presence of an organic sulfonic acid helps eliminate formation of N-benzyl compounds and suppresses certain other undesirable side reactions.